Thursday June 19 3:39 PM EDT

-- Women who take calcium after menopause to reduce their risk of the bone-thinning disease osteoporosis may be increasing their risk of zinc deficiency, a new study suggests.

Taking a calcium supplement with a meal can reduce zinc absorption by 50%, according to a report in the current issue of the American Journal of Clinical Nutrition. Already, about half of older women consume less than the recommended dietary allowance (RDA) for zinc, which is 12 milligrams per day for women over age 50.

"This observation suggests that elderly women may be particularly vulnerable to factors that can alter zinc bioavailability or the requirement for zinc, such as a high-calcium diet as shown here," reported lead author Richard Wood, of the Mineral Bioavailability Laboratory at Tufts University in Boston.

While Wood and colleague Jia Ju Zheng did not look at whether a high-calcium diet led to a zinc deficiency over time, lack of zinc can cause loss of appetite, skin changes and immunologic abnormalities, said Wood. However, consult your doctor before making any dietary changes, because it is possible to have too much of a good thing. Taking three to four times the RDA of zinc can cause alteration in immune function, as reported previously by Reuters.

During the one-month study, 18 women aged 59 to 86 increased calcium intake by 468 milligrams of calcium a day, either by drinking more milk or taking a supplement, and had a drop in zinc absorption of 2 milligrams per day. While the women normally absorbed about 13% of the zinc in their diet, that dropped to 1% to 2% when they were drinking more milk or taking a calcium supplement.

In a second study of men and women aged 21 to 69, the researchers found that zinc absorption dropped by 50% when they took a 600 milligram calcium supplement with a single meal. However, that drop could be offset by taking an extra 7.8 milligrams of zinc as part of the supplement.

"Our findings clearly suggest that consumption of a high-calcium diet can significantly reduce net zinc absorption and zinc balance in postmenopausal women and increase the risk of developing negative zinc balance," Wood wrote. "It seems prudent that vulnerable groups, such as elderly people with low zinc intakes, should increase their zinc intake if they increase their calcium intake substantially," the author concluded.

In 1994, the National Institutes of Health Consensus Development Conference on Optimal Calcium Intakes recommended that all people over 65 take 1500 milligrams of calcium a day to reduce the risk of fractures due to bone-thinning and osteoporosis.

The evidence that calcium reduces zinc in other people has been conflicting, according to the report, although one study in young men found that high levels of calcium did not influence zinc absorption.

"Given the increased popularity of calcium supplementation and a general trend toward recommending increased calcium intake for most of the population, additional study of the effects of high-calcium diets on zinc balance in various groups, especially in growing girls and young adult females, appears to be warranted," the authors wrote.

SOURCE: American Journal of Clinical Nutrition (1997;65:1803-1809)